Histamine H₁-Receptor-Mediated Breakdown of Inositol Phospholipids in Primary Neuronal Cultures

Abstract

Histamine stimulation of phosphoinositide hydrolysis was measured in primary cultures of neurons obtained from cerebrum of fetal rats of 16 days gestation. In cells prelabeled with (³H)inositol, histamine in the presence of lithium induces accumulation of (³H)inositol monophosphate linearly for 30–40min. The effect is concentration-dependent (Emax=149±9% over basal, EC₅₀=5,8±0,9 μM, Hill coefficient=0,99±0,07, n=3) and shows pharmacological characteristics typical of an H₁-mediated response. The histamine effect increases with neuronal differentiation during the first week in culture and stabilizes thereafter. In contrast to astrocytes where the histamine response is abolished when Ca²⁺ is omitted from the incubation medium, in neurons it is necessary to quelate all the remnant ion to prevent the effect. Furthermore, the response to a depolarizing concentration of K⁺ is additive to that of histamine and the stimulation by the amine is not affected by omission of Na⁺, or by incubation with the voltage-dependent Na⁺ channel blocker tetrodotoxin. All these observations indicate that the H₁-receptor-stimulated inositol phosphate formation in neurons is direct and not a consequence of a histamine-induced depolar ization, increased Ca²⁺ permeability or release of other neurotransmitters.